1. Field of the Invention
The present invention relates to a method for manufacturing a vertical cavity surface emitting laser and a multiple wavelength surface emitting laser, a vertical cavity surface emitting laser, a multiple wavelength surface emitting laser, and an optical communication system. In particular, the invention relates to a method for manufacturing a vertical cavity surface emitting laser and a multiple wavelength surface emitting laser, a vertical cavity surface emitting laser, a multiple wavelength surface emitting laser, and an optical communication system, with improved wavelength reproducibility and excellent mass productivity.
2. Description of the Related Art
In the high-speed optical communication, WDM (Wavelength Division Multiplexing) for multiplexing the wavelengths of communication semiconductor lasers and transmitting them has been widely used. In the present circumstances, however, WDM is limited to the use in the backbone or metro area network and has not reached the practical use stage for use of office or home.
However, in recent years, it has become possible to perform the high-speed optical communication using an optical fiber even in office or home. By FTTH (Fiber To The Home), a high-speed communication network at maximally 100 Mbps has begun to be served and introduced in SOHO (Small Office Home Office). In general, the optical fiber is drawn into offices or residences via utility poles and the link from the station. Optical signals transmitted to the office or home by the optical fiber are converted into electric signals using ONU (Optical Network Unit) placed in the home or in the vicinity thereof, thereby constructing LAN (Local Area Network) utilizing an Ethernet (a registered trademark) system and achieving communication with PC (Personal Computer). In this case, PC is connected to HUB by a metal UTP (Unshielded Twist Pair) cable.
On the other hand, in constructing LAN, a system using all optical networks without metal UTP cables, etc. is also being studied. In particular, in the short-range optical communication in an office, etc., studies on the wavelength division multiplexing system using POF (Plastic Optical Fiber), which is cheaper than a quartz based optical fiber, are advancing. This is because the capacity of communication information becomes large and it is demanded to more easily share motion images or large-capacity databases.
In such a demand, VCSEL's (Vertical Cavity Surface Emitting Laser) play an important role as optical devices. VCSEL's are different from edge emitting semiconductor lasers broadly used and can providelight emitting from the surface of a semiconductor substrate. In this way, since many elements can be prepared simultaneously on the same substrate, it is possible to obtain light sources which are strongly in favor of mass productivity, that is realization of low costs and have stable performance.
In VCSEL, for example, an active layer is sandwiched between DBR (Distributed Bragg Reflector) structures formed of a semiconductor multilayered film to reflect light, thereby realizing an optical resonator. Usually, the semiconductor multilayered film includes several tens semiconductor layers, and by changing a composition ratio of AlGaAs, the DBR structures are formed. Light is usually output from the upper DBR side, and 100% of the light is reflected in the lower DBR. On the other hand, in the upper DBR portion, for the purpose of picking up light, the reflectance is designed to an extent of about 99%, thereby picking up the light. The reason why the reflectance is high in this way resides in the matter that VCSEL has a very narrow thickness of the active layer as about 50 nm due to its structure so that it is difficult to obtain a high gain. Also, in general, an MQW (Multi Quantum Well) structure is employed in the active layer.
The DBR structures are simultaneously prepared over the whole of the substrate by the metal organic chemical vapor deposition (MOCVD) method. For that reason, the oscillation wavelength is entirely the same over the whole of the substrate. However, as described previously, when used in the wavelength division multiplexing system, it is necessary that plural VCSELs provided adjacent to each other on the same substrate have wavelengths different from each other.
As to such a multiple wavelength surface emitting laser, for example, there has been proposed a method for preparing a mesa structure in which the diameter is different among the adjacent VSCELs on the substrate and then preparing DBR structures or light emitting layers by the MOCVD method (for example, see IEEE Photonics Technology Letters Vol. 7, p. 10-12 “Wavelength control of vertical cavity surface-emitting lasers by using nonplanar MOCVD” Koyama, F.; Mukaihara, T.; Hayashi, Y.; Ohnoki, N.; Hatori, N.; and Iga, K.). In such VSCELs, the film thickness changes at the time of film formation by MOCVD, depending upon a difference in the size of the mesa of the substrate to cause a change of the film thickness structure of DBR or MQW, whereby the oscillation wavelength changes.
Also, there has been known a surface emitting device having a construction prepared by successively laminating a lower clad layer, an active layer, an upper clad layer and an upper reflection film on a lower reflection film and forming a wavelength tuning layer made of a material whose refractive index changes by a chemical change on the upper clad layer (JP-A-Hei. 9-135051 (paragraphs 0013 to 0022; and FIG. 2)).
Also, it has been known that when a dielectric mask such as SiO2 is grown in the doughnut shape in at least a part of the light emitting region by, for example, the MOCVD method or CBE method, the growth speed and composition of a crystal growing in an opening at the center can be changed in accordance with the width (wall thickness) of the doughnut shape, whereby the cross-sectional shape, the growth film thickness, the oscillation wavelength, and the like can be flexibly designed (JP-A-Hei. 10-233559 (paragraph 0034; and FIGS. 1 and 2).
As described above, various researches regarding VCSEL are advancing, and it is considered that light sources for high-speed communication using VCSEL will become important in the future. Taking into account such backgrounds, it is considered that realization of high-speed communication technologies, which can be employed in offices and homes, is important. Then, development of technologies of an optical communication system using an optical fiber such as POF is advanced.
POF has such a characteristic feature that POF can be easily connected and used. It has been proven that when POF is combined with VCSEL, communication of 2 GHz or more can be achieved so far as the communication distance is short. POF is used in the multiple mode, and its core size is several hundreds microns and large as compared with quartz based single mode fibers. Therefore, though POF has such characteristic features that alignment between fibers can be easily performed and that its handling such as connection and removal is easy, its transmission loss is large and its transmission distance is restricted. In general, it is considered that the transmission distance is about 50 m and that the transmission speed is limited to several Gbps. However, if VCSEL emitting light of multiple wavelengths is realized and is applied to POF, wavelength division multiplexing communication becomes possible, and ultrahigh-speed transmission can be realized.